Method of forming and maintaining offshore ice structures

ABSTRACT

The specification discloses a method of constructing and maintaining an ice structure at a desired, frigid, offshore location which can be used for drilling and/or producing oil wells. An ice floe or a part of a fast ice mass forms the base on which ice is accumulated to form the structure. The ice can be accumulated by spraying, flooding, or piling up of ice. The structure may be reinforced and has means to protect it from marginal melting during the &#39;&#39;&#39;&#39;summer&#39;&#39;&#39;&#39; months.

United States Patent 1 Fitch et a1.

[ METHOD OF FORMING AND MAINTAINING OFFSHORE ICE STRUCTURES [75]Inventors: John L. Fitch; Lloyd G. Jones, both of Dallas, Tex.

[73] Assignee: Mobil Oil Corporation, New York,

[22] Filed: Oct. 19, 1970 [21] Appl. No.: 81,940

[52] US. Cl 61/46, 61/63, 62/1, 62/64, 62/259 [51] Int. Cl. F25c l/02[58] Field of Search 62/1, 260, 66, 67, 62/74,-75, 259; 61/36 A, l, 46,63

[111 3,750,12 [451 Aug. 7, 1973 Alaskan Ports, Civil Engr. in the Ocean11 Asce Conf. Miami Beach Fla. 12/10-12, 1969.

Field solidification & Deshlination of Sea Ice, Adams et al.,Proceedings of M.l.T. Conference of 2/12-16, 1962.

Ice Islands Studied for Artie Oil Work, Dallas Morning News 8/15/70.

lce Reinforcement, Coble et al., Proceedings of M.l.T. Conference 1962,p. 130.

Primary Examiner-William E. Wayner Attorney-William J. Scherback,Frederick E. Dumoulin, Drude Faulconer, Andrew L. Gaboriault and SidneyA. Johnson [57] ABSTRACT The specification discloses a method ofconstructing and maintaining an ice structure at a desired, frigid,offshore location which can be used for drilling and/or producing oilwells. An ice flu: or a part of a fast ice mass forms the base on whichice is accumulated to form the structure. The ice can be accumulated byspraying, flooding, or piling up of ice. The structure may be reinforcedand has means to protect it from marginal melting during the summer"months.

7 Claims, 14 Drawing Figures PATENTED M18 7 3. 750 .41 2

SHEET 1 0F 4 l I E Illilk 3/ 3! 31' 5 a he; "a; 52": 5% a in? JOHN L.FITCH LLOYD G. JONES INVENTORS ATTORNEY PATENTED 3. 750.41 2

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TEMPERATURE OF FIG. I

FREEZING POINT /MAXIMUM DENSITY JOHN L. FITCH SALINITY OF AVERAGE SEAWATER LLOYD G. JONES INVENTORS I mmiqmmasmk SALINITY,

AT TORNE Y PATENTEU 7 I973 SHEET 0F 4 FIG. IOA

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ATTORNEY METHOD OF FORMING AND MAINTAINING OFFSHORE ICE STRUCTURESBACKGROUND OF THE INVENTION This invention relates to a method ofproviding a structure in a hostile, frigid environment and moreparticularly relates to a method of building and maintaining an iceisland at a frigid offshore location.

The increasing demand for petroleum products has required theexploitation of many new regions throughout the world. One of the mostpromising of these regions from the standpoint of potential reserves isthat which lies along the Artic shelf which stretches some 3,000 milesfrom western Alaska to eastern Greenland. While there is little questionthat vast amounts of petroleum are present in this area, the productionof this petroleum presents several new technical and economicalproblems.

The Artic shelf in many places is broad with a gentle slope. In winter,ice attaches itself to shore and freezes outward as much as fifty milesfrom shore. This fast ice could possibly serve as a temporary, stableplatform for drilling or other operations but unfortunately this icebreaks up in the summer and shore-leads of open water develop throughthe ice. Some of these leads may range up to 200 miles in width. Whilethe leads are open, floating drilling operations might be carried outbut the risk is great since pack ice frequently moves shoreward underthe influence of winds and currents. When this happens, the pack ice cancompletely close the shore-leads, thereby damaging any equipment withinthe leads. Further, this pack ice, I

which may range up to ten feet or more in thickness, can exert massiveforce which may be too great to be resisted by any practicaldrilling/production fixed platform of the conventional type. Therefore,to operate successfully in these areas of the Artie shelf, thedrilling/production structure must be capable of withstanding oravoiding the force of moving pack ice.

One structure which can successfully resist such forces is an islandwhich extends upward from an anchored position on the marine bottom to adistance above the waterline. ldeally, this island would be a naturallyoccurring one but unfortunately such islands are not normally present inthis area or are located in the wrong places to serve a particularfield. It follows that, in most instances, if an island is to be used,it must be of some other form.

Three alternate forms of such islands are as follows: (1) artificialislands built of earth materials and the like; (2) natural ice islands;and (3) artificial ice islands. As to artificial earth islands, thesevere shortage or difficulty of obtaining the required materials in theArctic areas makes their use impractical in most of these areas. The useof natural ice islands in these areas has been investigated asreportedin OIL AND GAS JOURNAL, July 28, 1969, pp. 118-119, but these attemptswere abandoned due to severe cracking of the islands. A furtherdifficulty is that natural ice islands are unlikely to be present at thedesired location and time. This leaves artificial ice islands to whichthe present inventionrelates.

There are several important considerations involved in providing anartificial ice island which is capable of serving as a year-round,permanent, or semipermanent offshore structure. First, the base of theisland has to be selected and must be properly positioned at a desiredlocation. Second, ice has to be accumulated onto the base to actuallyform the artificial island. Third, the island must be attached to thesea bottom to a degree sufficient to substantially prevent it frommoving laterally under forces imposed by drifting ice, wind, andcurrents. Fourth, the island must have sufficient mechanical strength toresist major breakup due to forces imposed by ice, wind tides, waves, orother forces. Last, the island has to be maintained during periods whenthe ambient temperature of the water and/or air rises above the meltingpoint of the ice forming the island so that the island will not melt orbreak up.

SUMMARY OF THE INVENTION The present invention provides a method forconstructing and maintaining an ice structure at a desired, frigid,offshore location which satisfies the considerations mentioned above.

A base of naturally occurring ice which is normally a relatively flatslab of ice is selected and anchored over the desired location. Thisbase may be an ice floe or it may be part of a fast ice mass normallyoverlying the desired location. Reinforcing material may be provided onthe base to strengthen it and to give it added weight for stability. Inone embodiment, piles are passed through the base and into the marinebottom to anchor the base and to aid in construction of the structure.

Ice is then accumulated on the base. Accumulation of ice may be byfreezing water which is sprayed or flooded onto the base, bydistributing crushed ice about the base, or by piling up blocks of iceon the base which are gathered from the surrounding environment. Sinceconstruction of the structure takes place in the Arctic winter, norefrigeration equipment is needed to freeze the water. The ice isaccumulated so that the sides of the structure are substantiallyvertical or slope slightly inward. Reinforcement may be addedperiodically during accumulation of the ice to maintain sound structuralintegrity of the structure. The base under the added weight of theaccumulated ice will begin to sink toward the marine bottom.Accumulation of ice is continued until a structure is formed whichreaches from the marine bottom to above the waterline.

The structure is protected from marginal melting during the summermonths by several different methods. One method provides a thermalbarrier about the sides of the structure in the form of insulation.Another method provides a barrier about the structure which traps waterfrom surface melting against the sides of the structure to prevent thesurrounding sea water from contacting the structure. Another providesrefrigerating the structure to further reduce the temperature of the iceforming the structure. Still another method provides for bringing colderwater from its normal depth to the surface adjacent the structure toreduce the temperature of the surface water which contacts the marginsof the structure. Also, sacrificial blocks of fast ice or pack ice canbe secured to the margins of the structure and allowed to melt as thewater temperature rises to keep the temperature of the water reducednear the margins.

The above-mentioned and other advantages of the invention will he morereadily appreciated as the invention becomes better'understood byreference to the following detailed description when considered inconnectlon with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a perspective view of avessel towing an ice floe to a desired offshore location;

FIG. 2 is a perspective view of an initial stage of construction of thepresent invention;

FIG. 3 is a plan view of a base of ice utilized in the presentinvention;

FIG. 4 is a cross-sectional view taken along section line 4-4 of FIG. 3;

FIG. 5 is a cross-sectional view of a sprinkler system utilized in thepresent invention;

FIG. 6 is a cross-sectional view of the sprinkler system of FIG. 5 at alater stage of the present invention;

FIG. 7 is a perspective view of an ice structure constructed inaccordance with the present invention and having marginal protectionmeans associated therewith;

FIG. 8 is a graph displaying the temperature versus depth which istypical in the Beaufort Sea during the summer;

FIG. 9 is a perspective view of an ice mass after partial melting;

FIG. 10A is a cross-sectional view of an ice structure having one formof marginal protection in accordance with the present invention;

FIG. 10B is a cross-sectional view of an ice structure having anotherform of marginal protection in accordance with the present invention;

FIG. 11 is a graph displaying temperature of maximum density and offreezing point versus salinity of water;

FIG. 12 is a perspective view, partly in section, of one type ofinsulative barrier which may be used in the present invention; and

FIG. 13 is a perspective view, partly in section, of another type ofinsulative barrier which may be used in the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In constructing an ice island,a base having the desired surface area, e.g., about one acre or greater,is first selected. This base, which will normally be a relatively fiat,thin sheet of ice, e.g., five feet thick, may be an ice floe or it maybe part of the fast ice mass. If an ice floe is used, a single floehaving the desired surface area is preferred, but if such a floe isunavailable, several floes may be grouped and frozen together to formthe base. The floe or floes will likely have to be pushed or towed tothe desired location. If towed, a gridlike means 11 (FIG. I) made ofreinforcing steel or the like is positioned on floe l0 and water pumpedor sprayed onto floe 10 to freeze means II in place. A towline 12 fromvessel 13 is connected to means 11 so that floe 10 can be towed toposition. The means 11 is preferably left in place to providereinforcement of the base as will be discussed in more detail below.

When floe 10 is in position, it is anchored to the marine bottom byregular mooring lines having anchors thereon or by other suitableanchoring techniques. One such technique is the same as that shown inFIG. 2 wherein the base is formed from fast ice which overlies thedesired location of the island. Referring more particularly to FIG. 2,the desired areal extent of base 22 is first marked off(as representedby dotted line 23) on fast ice which is frozen to shore 21. If it isnecessary to provide a means for maintaining the island in the desiredlocation during construction, other than that provided by attachment ofthe ice to the shore, holes are drilled or otherwise provided throughbase 22 and pilings 25 are passed through these holes and are drilled,driven, or otherwise affixed into the marine bottom. By circulatingrefrigerant through the piles, they can be frozen into the marine bottomto increase resistance to lateral movement of the structure. Threepilings 25 are illustrated but the actual number required will be determined by engineering considerations, e.g., size and strength of thebase, etc. These pilings not only serve to keep the island in placewhile it is being constructed, but also add additional support to theisland once it is completed.

With base 10 or 22 in place, the construction of the island, itself, cancommence. In some instances, the base may be strong enough, as is, toallow construction of the island. Preferably, however, reinforcing willbe provided for the base to add strength thereto. This reinforcing alsoprovides added weight to the base which aids in preventing the islandfrom tipping during construction. Techniques similar to those employedin reinforcing concrete can be used. A preferred reinforcing arrangementis shown in FIG. 3 wherein lengths 27 of reinforcing steel radiateoutward from the center of base 10 or 22 and are connected together bycross lengths 28. Any reinforcing material, e.g., steel rods, pipe,cable, mesh, fiber, etc., can be used as the situation dictates.Discarded drill pipe is especially attractive and can be assembled tomaintain fluid communication between lengths of pipe so a refrigerantcan be circulated therethrough, as will be discussed more fully below.

Since ice will normally slip or slide on fine-grained, marine bottomsediments under the influence of pressure from the ice pack or movingfloes, it may be necessary to provide some means for preventing the iceisland, once formed, from slipping on the bottom, especially if piles 25are not employed. One such means involves laying a layer of coarsegravel and rock 32 (FIG. 4) on the marine bottom underlying the base 22.Another means is to implant spaced lengths 31 of pipe (only a few shownin FIG. 4) through base 22 so they extend a substantial distance belowthe base and will act as "spikes when the bottom of the island engagesthe marine bottom. Still another means is to freeze" the island to thebottom with refrigeration, as will be more fully described below.

With base 10 or 22 completed, ice is next accumulated on the base toactually form the island. in the present invention, it is contemplatedthat the building of the island will take place during the "winter"season of the year so that ambient temperatures are sufficiently belowthe freezing point of water. For example, the average mean temperaturefor areas overlying the Arctic Shelf from around the first of Dec. tothe 15th of Mar. is about ISF., with subfreezing temperature extendingfrom Sept. to June. Construction of the island is carried out duringthis period so that accumulation of ice will not require refrigerationequipment to freeze ice on the island.

The accumulation of ice can be achieved in several different ways orcombinations thereof. Preferably, a sprinkler system is provided toallow water to be sprayed onto the base. By spraying fine drops of waterthrough the frigid air, freezing is significantly enhanced and rapidaccumulation of ice can take place. A typical sprinkler system 40 isillustrated in FIGS. 5 and 6. Pump 41 is positioned on fast ice and hasintake 42 extending through ice 20 to the underlying water. Outlet 43 ofpump 41 is connected to the interior of jacket 44 which is anchored in avertical position on fast ice 20 by support 45. Jacket 44 is closed atits upper end and open at its lower end. Standpipe 46 extends upwardthrough the lower end of jacket 44 and is connected to a manifold 47 ofpiping which extends over the surface of base 22. A seal means 48 isprovided in jacket 44 to allow sliding movement between the seal andstandpipe 46 while at the same time sealing the lower end of the annulusbetween jacket 44 and standpipe 46. A plurality of outlet pipes 49 areconnected to manifold 47 and each has a rotating sprinkler head 50 atthe upper end thereof. The pipes 49 are strategically spaced on base 22so that when the sprinkler system is in operation, water will be sprayedin such a manner to provide substantially equal accumulation of ice onthe base. A typical pattern is represented by the dotted lines 30 (FIG.3). in the sprinkler system illustrated, other connecting means could beused in place of the jacket and sliding seal to connect-the pump outletwith the standpipe, e.g., a length of flexible hose.

To prevent the water from freezing in the system, the piping isinsulated where exposed to subfreezing temperatures, as required.Further, sprinkler heads 50 are preferably coated with or made ofnonsticking material, e.g., polytetrafluoroethylene, so that thecentrifugal force of the rotating heads will sling off the ice as thewater is exposed to the air. Still further, heating means (not shown),e.g., circulating hot fluids or electrical heating, can be provided ateach sprinkler head and at jacket 44 to prevent water from freezingprematurely. Further, a small pipe (not shown) is concentricallypositioned throughout the piping of the system through which anantifreezing mixture, e.g., methanolwater, may be circulated to preventfreezing or to thaw any ice that may form in the pipes.

in operation, pump 41 pumps water into jacket 44 where it fills theannulus above sea] 48 and overflows into standpipe 46. The water flowsfrom standpipe 46, into manifold 47, through outlet pipes 49, and outsprinkler heads 50. As the ice formed from the sprayed wateraccumulates, base 22 will begin to sink under the added weight.Standpipe 46, being slidable through seal 48, will allow manifold 47 andoutlet pipes 49 to sink with base 22. When ice nears the top of outletpipes 49, pump 41 is stopped and sprinkler heads 50 are removed so newlengths of pipe 46a, 46b, 49a, 49b, can be added. Heads 50 are replacedand operation is resumed. Just before pumping is ceased, all water maybe pumped out of the system to prevent freezing while the lengths ofpipe are added. When ice floe Ml) is used as the base, pump 41 would bemounted adjacent the floe on an anchored barge or the like.

Another accumulation technique is merely flooding the base with waterand allowing it to freeze. it must be remembered, however, that there isa great deal of localized brine concentration when salt water is frozen.This leads to brine inclusions and weakness in the ice, plus a tendencyfor the ice to melt at a slightly lower temperature than fresh waterice. in accumulating large quantities of ice by flooding or by sprayingsea water through the air, the concentrated brine produced by thefreezing process cannot escape downward to the sea in the normal fashionsince the existing ice layers block rapid migration of brine. Therefore,in order to produce ice with low salt content, some means of producingbrine flow from the accumulated ice must be provided.

Much of the excess brine can be removed from the ice by occasionallyflooding the new ice with an excess of sea water. The sea water is about3.5 per cent salt while the locally concentrated brine will be at muchhigher levels of salt content. Therefore, by flooding with an excess ofsea water, the localized brine concentration in the new ice can belowered to approximately 3.5 per cent, leaving the overall salt contentof the accumulated ice at a level considerably below 3.5 per cent. inthe flooding process of ice accumulation, the excess sea water is addedperiodically, e.g., every hour or so, allowing the concentrated brine tospill out onto the ice pack surrounding the island. in the sprayingprocess of ice accumulation, the excess sea water is added (sprayed)periodically, e.g., every day or two, or more often if necessary.

Another useful method for inducing brine flow is to adjust the sprayingrate so that all of the water does not freeze, leaving a small amount torun off into the sea. The granular accumulation of ice pellets is porousenough to allow the excess sea water to flow down through the top of theisland and out at the sides onto the surrounding ice pack or into thesea water by gravity drainage, forcing out before it the concentratedbrine located internally in the island structure as a result of thefreezing process.

Although sea water will likely have to be used due to availability, itshould be recognized that fresh water is preferred since it has a higherfreezing/melting point than salt water and forms stronger ice.

in some instances, the depth of water may be too great to allow an iceisland having the necessary height to be formed by means of spraying orflooding during the time available. In these instances, ice, itself, maybe accumulated rather than freezing water to form the ice. This can becarried out in several ways. One way utilizes an ice crusher which ismounted on a barge if base It) is surrounded by open water or on amobile carrier if base 22 is surrounded by fast ice. lce floes or packice is fed to the crusher (not shown) from which it is broken up andblown or otherwise spread onto base 10 or 22.

Another way to accumulate ice is to pull ice floes or blocks of ice 35which have been mined" from surrounding sea surface onto the base. Awinch 33 (FIG. 4) which is repositioned during accumulation is mountedon the base for this purpose. A gridlike anchor, such as means 111 inFIG. 1, is frozen into each floe or block to secure winch line 34thereto. The added ice is distributed on base 10 or 22 in such a way asto prevent the base from breaking up. A somewhat higher, uniform loadingis normally provided near the edge of base 10 or 22 than that in themiddle. Water is periodically flooded into the cracks between blocks ofice and allowed to freeze to weld the blocks together. Also, reinforcingmaterial may overlap adjacent blocks for added strength.

Further, it is desirable to distribute the ice during accumulation insuch a way as to provide the island with sloping sides (FIG. 5).Sprinkler heads 50 can be adjusted during accumulation to achieve thisresult if a sprinkler system is used. This is done in order to preventtensile forces from developing in the ice mass. If the part of the iceabove the waterline is allowed to project (overhang) beyond the lowerpart of the ice mass, there will be a tendency for this part of the iceto crack and fall off (spall off) into the sea. Thus, it is desirablefor the ice above the waterline to slope inward. Also, if the ice belowthe waterline is allowed to project for a substantial distance beyondthe average profile, buoyant forces, due to the density differencebetween ice and water, will tend to cause tensile cracks to develop inthe lower parts of the mass with resulting breakage of the mass andfloating of the broken blocks to the surface. Thus, it is desirable thatthe ice below the waterline have a substantially vertical profile or agentle inward slope. In some instances, however, it may be permissiblefor the sides to slope either slightly inward or outward by an amountdependent on the length of the slope and the combined strength of thenatural ice mass plus that provided by any reinforcing material whichmay have been added. As mentioned above, reinforcing material can beused with any of the accumulation techniques and can be addedperiodically in any necessary pattern to the island as it is beingformed.

As ice is added to base or 22, it will begin to sink under the addedweight. Where base 22 is formed from the pack ice, it may be necessaryto cut or'notch the ice about the periphery at the beginning of theaccumulation process. If piles 25 are used, it will be necessary toprevent the sides of the openings through base 10 or 22 from freezingthereto. This may be done in several ways. One way is to heat the piles,e.g., circulate warm water therethrough. Another way is to coat thepiles with a nonsticking substance, e.g., polytetrafluoroethylene, priorto positioning them into the marine bottom. Still another, is to useenlarged openings 29 (FIGS. 3 and 4) about each pile and fill theannulus 290 between the opening and pile with a nonsticking, nonfreezingsubstance, e.g., oil, methanol, etc. For this purpose, the organicsubstances, e.g., oil, are preferable since they continue to float and,by periodically filling the annulus, continuous nonfreezing protectionis provided around the piles as the island is being formed. If itbecomes desirable to freeze the piles to the island after it iscomplete, this substance can easily be pumped out and replaced withwater.

The island (FIG. 7) will slowly take shape as more and more ice is addedto the top until base 10 or 22 contacts the marine bottom. In order toinsure that there is sufficient weight to overcome buoyancy factors andresist forces imposed by wind, waves, and drifting ice packs, the islandshould substantially extend above the waterline. It has been determinedthat the distance above the waterline should be approximately 10 percent or more of that distance from the waterline to the marine bottom,depending on the overall density of the island which includes anyreinforcing material or materials, e.g., sand, gravel, etc., added forstability. In any case, the island should extend far enough above thewaterline to protect any drilling/production equipment 16 mountedthereon from damage by waves or ice and to allow for any surface meltingthat might occur in the summer.

The temperature of most of the water mass in the Arctic seas is near thefreezing point the year round. In midsummer, however, the surface watersare warmed by the sun. A typical Aug. temperature profile in open waterin the Beaufort Sea is shown in FIG. 3. In Sept.,

surface cooling begins, causing thermal convection to occur and theentire water mass approaches the freezing point temperature. At thistime, new ice begins to form on the sea surface. Thus, protection frommelting is required only during the brief summer period.

Melting may be conveniently considered in three zones: (1) in thesubaerial zone (above the water), where heating is mainly due to directabsorption of radiant energy from the sun, (2) in the sub-marine zone(water-ice contact area) where heat is transferred from the water to theice, and (3) in the bottom contact zone where heat is transferred fromthe sea floor to the ice.

Subaerial melting can be controlled or prevented by various well-knownmethods. One is by simply shading the surface from the sun. Another isto cover the surface with a layer of insulating (and reflecting)material such as polyurethane foam, commercial building insulation, etc.A third is to prevent accumulation of water on the surface. It is wellknown that surface melting of ice in the Arctic is very slow if a layerof water is not present. This is due to the fact that an ice surfacereflects most of the suns energy and very little heat is absorbed.

Sub-marine marginal melting or that melting of the islands perimetersdue to heat transfer from the sea water is critical and must becontrolled if the island is to be maintained for an extended periodthrough the summer. The melting and consequential breakup of an iceisland is similar to that of an iceberg and is illustrated in FIG. 9.The perimeter of ice mass will melt or erode as represented by cavities56 due to heat transfer and action of the surrounding sea water. Whentension cracks (dotted lines 57) occur in ice mass 55, large chunks 58of ice will break away. If the island is to serve only as a temporarystructure, e.g., for drilling a wildcat well, it may be preferable tosimply build an island large enough to serve its purpose before it isflnally consumed. However, when the island is to be of a permanentnature, some means must be provided to prevent melting and breakup ofthe island during the summer" period. As will be discussed below, thereare several methods or techniques of protecting the island.

One method of preventing marginal melting involves providing a layer ofthermal insulating material extending from above the point of vigorouswave action to a depth where the water temperature is substantially atthe melting point of the ice or to the marine bottom, if desired. It isonly necessary that the insulation layer provide a sufficient thermalbarrier. This is easily achieved because only a small temperaturedifferential exists between ice and the water, e.g., about 5F., andbecause most of the heat transfer is due to water motion past the icesurface induced by waves and currents which create a very thin boundarylayer through which heat transfers rapidly. Thus, any barrier whichrestricts water motion near the ice, thus thickening the boundary layer,will greatly retard melting.

One such barrier consists of a layer of rocks 60 placed about the islandas shown in FIG. 10A. As set out above, the sides of the island 15A maybe shaped, as the island is being constructed, so that they normallyslope inwardly from the bottom toward the center of the island. Thelayer of rock is deposited on this slope all the way around the island.The thickness of the rock layer and its angle of inclination is suchthat the inward stress component of the rock layer will equal or exceedthe net horizontal stress in the main ice mass of the island. Thisprevents radial creep of the ice and reduces the tendency of the islandto crack and break up.

Another typical barrier means 61a (FIG. 12) consists of a layer ofplastic foam 80, e.g., one-quarter inch of polyurethane supported on oneor both sides by tough plastic sheet material 81, such as polyethyleneor Mylar. The foamed insulating material is preferably of the closedpore type which retains its insulating properties when exposed to water.

Still another insulative barrier 61b (FIG. 13) is one of a quilted"sandwich construction wherein foamed insulation 82 is trapped betweentwo plastic sheets 63, 84 which in turn are joined in a quilted pattern.The water-impervious, plastic sheet on both sides adds strength andprotects the insulation from water. if holes develop in the plasticsheet only the cells with holes lose their insulating properties, theothers remaining undamaged.

Where the barrier is a sheet of insulative material as described above,it may be positioned from a vessel sailing around the island in a mannersimilar to that used by commercial vessels in laying fish nets or it maybe placed from the island by merely sliding it over the side. As shownin FIG. B, sheet 61 of insulation is positioned about the sides ofisland 15B. An anchoring or weight means 62, e.g., chain, is provided atthe lower end of sheet 61 to hold it in place while a pucker string 63or cable is used at the upper end of the sheet to hold sheet 61 to theisland. Since melting must be protected against only in the "summer,"sheet 61 can be removed and repaired during the winter months.

Because the sea water temperature in some locations, even at the surfaceduring summer, is generally only a few degrees above the ice meltingpoint (see FIG. 6), melting may be greatly inhibited by simplypreventing access of sea water to the island mass. This may beaccomplished by means of a plastic barrier similar to sheet 61 in FIG.108 except that insulation is not required. The plastic barrier, beingimpervious to water, prevents contact of the saline sea water with theice. Melting of the surface of the island, a moderate amount of which isdesirable in this case, produces relatively fresh water which drains offthe island behind the barrier and flows out at the bottom, therebypreventing encroachment of the sea water behind the barrier from below.As noted from chart in FIG. 11, the density of sea water is greater attemperatures below the freezing point, which gives the temperature ofthe Arctic seas a tendency to be at or near the freezing point at alldepths. Also, since the sea surface temperature for most of the time isapproximately 30F., the ice behind the barrier in contact with "fresh"water will not melt, whereas it would melt if the barrier were notpresent.

Another method of preventing marginal melting of the island is toutilize the naturally occurring environmental conditions of the frigidsea. Since the Arctic sea water temperature at depths greater than aboutfeet is always at approximately the freezing point (FIG. 6), melting ofthe island margin will be greatly inhibited if deep water is brought tothe surface near the island. This is done by means of a bubbler system76(FIG. 7 wherein air is pumped from air source '71 through line 72 intosubmerged pipe 73 which surrounds the structure. Pipe 73 is perforatedto permit formation of bubbles which causes the density of water todecrease and allows the colder water to rise. Such bubbler systems havebeen used in more temperate environments to circulate warm water to thesurface to prevent freezing around dam and bridge supports. Anothersystem which may be used to circulate cold water to the surface is onewhich utilizes one or more large, electrically or hydraulically drivenimpellers 76 mounted on or near the marine bottom to "churn" the wateraround the island, thereby causing the cold water to rise.

Another method to protect the margins of the structure is to "capture"blocks of ice from pack ice or fast ice and secure them about themargins of the structure, e.g., block 76 (FIG. 7) which is secured tostructure 15 by cables 79. This ice is allowed to melt as thetemperature of the water rises but substantially delays contact of thewater with the structure.

Still another method of preventing melting and breakup of the island isto refrigerate the island so the ice mass is always below freezing. Thisis done by converting the sprinkler system, if one is used to constructthe island, into a refrigeration system. To convert system 410 to arefrigeration system, all water is pumped out of the system andsprinkler heads 50 are removed. Ambient air (during periods when the airis below the freezing temperature) is pumped through jacket 44,standpipe 66, and out pipes 49. Heat of compression of this air isremoved, if required by use of an after-cooler, using ambient air forcooling. If a positive refrigerant is circulated through the system, theoutlets of pipes 49 are manifolded together (not shown) to provide aclosed system for the refrigerant.

Further, holes 66 (only one shown in FIG. can be drilled in the islandafter it is complete, and refrigeration pipes 66 installer therein.Frigid air or another refrigerant is circulated down inner pipe 66 andup the annulus 67 between pipe 66 and either cased or open hole 66. Theamount of cooling required to lower the temperature of the ice mass afew degrees is relatively low. Further, since the cooling need only becarried out a short period during the year and ambient air can be usedas a refrigerant, the cost is relatively low. In addition to providingprotection from melting, an important added advantage results fromrefrigeration. By maintaining the island temperature slightly belowfreezing, the island will be self-healing. That is, if any cracks occurin the island, upon being filled with water the cracks will quicklyrefreeze. This is due to the fact that the heat capacity of the largecolder ice mass will easily supply the necessary sink for the smallamount of heat which must be removed for the water in the crack tofreeze.

No protection of the ice island from melting at the bottom is needed. Asmall amount of melting will normally occur soon after the islandcontacts the marine bottom but after steady-state conditions arereached, little melting will occur. Also, if refrigeration is used, suchas pipes 66 in FIG. 166, the island can be frozen to the marine bottomwhich further prevents bottom melting.

it should be recognized that one or more of the above methods can beused singly or in combination to construct a particular ice island andto prevent marginal melting thereof.

it should also be recognized that, although this description of theinvention applies to marine construction, parts of the invention may bepracticed at onshore locations. For example, in drilling for petroleumin onshore Arctic areas, it is necessary to protect the soil permafrostfrom melting in the vicinity of the drill site.

This is commonly accomplished by providing a thick pad of gravel in thearea to be protected. A much less expensive method of providing suchprotection from melting is to accumulate a pad of ice by the methodscited herein.

Still further, although the present invention illustrates the structureas being primarily for drilling and- /or production of oil, it should berealized that structures constructed and maintained in accordance withthe present invention may be put to other uses, e.g., dockingfacilities, storage areas, airstrips, etc.

What is claimed is:

1. The method of providing protection from marginal melting for an icestructure when the ambient temperature of the water surrounding saidstructure rises above the melting point of the ice forming saidstructure, said method comprising:

providing an insulative barrier around said structure to prevent contactof said water with said structure, said barrier extending from above thewaterline to at least the depth at which the temperature of the watersurrounding said structure is at or near the melting point of said iceforming said structure, said barrier comprising a sheet of plasticmaterial having polyurethane foam on both sides thereof.

2. The method of providing protection from marginal melting for an icestructure when the ambient temperature of the water surrounding saidstructure rises above the melting point of the ice forming saidstructure, said method comprising:

positioning a sheet of water-impermeable material about said structure,said sheet extending from above the waterline to at least the depth atwhich the temperature of the water surrounding said structure is at ornear the melting point of the ice forming said structure; and

allowing water resulting from the melting of the surface of saidstructure to flow from said surface in between said sheet and saidstructure.

3. The method of providing protection from marginal melting for an icestructure when the ambient temperature of the water surrounding saidstructure rises above the melting point of the ice forming saidstructure, said method comprising:

providing an insulative barrier around said structure to prevent contactof said water with said structure, said barrier extending from above thewaterline to at least the depth at which the temperature of the watersurrounding said structure is at or near the melting point of said iceforming said structure, said barrier comprising foamed insulationtrapped between two plastic sheets joined together in a quilted pattern.

4. A method of constructing an ice structure at a desired offshorelocation in a frigid environment wherein a base of naturally occurringice overlies said desired location, said method comprising:

accumulating ice on said base by spraying water onto said base and thesubsequently formedice whereby said base moves downward under the addedweight of the accumulated ice;

periodically spraying water at a rate in excess to the amount that willfreeze;

allowing the excess water to drain from the structure to aid in reducingthe salinity of the ice; continuing to accumulate ice on said base byspraying water thereon until sufficient ice has been accumulated on saidbase to form a structure which extends from the marine bottom to abovethe waterline; and

providing protection from marginal melting of said i structure when theambient temperature of the water surrounding said structure rises abovethe melting point of the ice forming said structure by placing a sheetof water-impermeable material around the periphery of said structurewhere said structure contacts the water.

5. A method of constructing an ice structure at a desired offshorelocation in a frigid environment wherein a base of naturally occurringice overlies said desired location, said method comprising:

' accumulating ice on said base by spraying water onto said base and thesubsequently formed ice whereby said base moves downward under the addedweight of the accumulated ice;

continuing to accumulate ice on said base by spraying water thereonuntil sufficient ice has been accumulated on said base to form astructure which extends from the marine bottom to above the waterline;

providing protection from marginal melting of said structure when theambient temperature of the water surrounding said structure rises abovethe melting point of the ice forming said structure by placing a sheetof water-impermeable material about the sides of the structure; and

allowing water resulting from the melting of the surface of saidstructure to flow from said surface in between said sides of saidstructure and said sheet.

6. A method of constructing an ice structure at a desired offshorelocation in a frigid environment wherein a base of naturally occurringice overlies said desired location, said method comprising:

accumulating ice on said base by spraying water onto said base and thesubsequently formed ice whereby said base moved downward under the addedweight of the accumulated ice;

continuing to accumulate ice on said base by spraying water thereonuntil sufficient ice has been accumulated on said base to form astructure which extends from the marine bottom to above the waterline;and

providing protection from marginal melting of said structure when theambient temperature of the water surrounding said structure rises abovethe melting point of the ice forming said structure by placing a sheetof plastic material having polyurethane foam on both sides thereofaround the periphery of said structure where said structure contacts thewater.

7. A method of constructing an ice structure at a desired offshorelocation in a frigid environment wherein a base of naturally occurringice overlies said desired location, said method comprising:

accumulating ice on said base by spraying water onto said base and thesubsequently formed ice whereby said base moves downward under the addedweight of the accumulated ice;

continuing to accumulate ice on said base by spraying water thereonuntil sufficient ice has been accumulated on said base to form astructure which extends from the marine bottom to above the waterline;and

structure contacts the water, said sheet of waterimpermeable materialbeing comprised of foam insulation trapped between two plastic sheetsjoined together in a quilted pattern.

1. The method of providing protection from marginal melting for an icestructure when the ambient temperature of the water surrounding saidstructure rises above the melting point of the ice forming saidstructure, said method comprising: providing an insulative barrieraround said structure to prevent contact of said water with saidstructure, said barrier extending from above the waterline to at leastthe depth at which the temperature of the water surrounding saidstructure is at or near the melting point of said ice forming saidstructure, said barrier comprising a sheet of plastic material havingpolyurethane foam on both sides thereof.
 2. The method of providingprotection from marginal melting for an ice structure when the ambienttemperature of the water surrounding said structure rises above themelting point of the ice forming said structure, said method comprising:positioning a sheet of water-impermeable material about said structure,said sheet extending from above the waterline to at least the depth atwhich the temperature of the water surrounding said structure is at ornear the melting point of the ice forming said structure; and allowingwater resulting from the melting of the surface of said structure toflow from said surface in between said sheet and said structure.
 3. Themethod of providing protection from marginal melting for an icestructure when the ambient temperature of the water surrounding saidstructure rises above the melting point of the ice forming saidstructure, said method comprising: providing an insulative barrieraround said structure to prevent contact of said water with saidstructure, said barrier extending from above the waterline to at leastthe depth at which the temperature of the water surrounding saidstructure is at or near the melting point of said ice forming saidstructure, said barrier comprising foamed insulation trapped between twoplastic sheets joined together in a quilted pattern.
 4. A method ofconstructing an ice structure at a desired offshore location in a frigidenvironment wherein a base of naturally occurring ice overlies saiddesired location, said method comprising: accumulating ice on said baseby spraying water onto said base and the subsequently formed ice wherebysaid base moves downward under the added weight of the accumulated ice;periodically spraying water at a rate in excess to the amount that willfreeze; allowing the excess water to drain from the structure to aid inreducing the salinity of the ice; continuing to accumulate ice on saidbase by spraying water thereon until sufficient ice has been accumulatedon said base to form a structure which extends from the marine bottom toabove the waterline; and providing protection from marginal melting ofsaid structure when the ambient temperature of the water surroundingsaid structure rises above the melting point of the ice forming saidstructure by placing a sheet of water-impermeable material around theperiphery of said structure where said structure contacts the water. 5.A method of constructing an ice structure at a desired offshore locationin a frigid environment wherein a base of naturally occurring iceoverlies said desired location, said method comprising: accumulating iceon said base by spraying water onto said base and the subsequentlyformed ice whereby said base moves downward under the added weight ofthe accumulated ice; continuing to accumulate ice on said base byspraying water thereon until sufficient ice has been accumulated on saidbase to form a structure which extends from the marine bottom to abovethe waterline; providing protection from marginal melting of saidstructure when the ambient temperature of the water surrounding saidstructure rises above the melting point of the ice forming saidstructure by placing a sheet of water-impermeable material about thesides of the structure; and allowing water resulting from the melting ofthe surface of said structure to flow from said surface in between saidsides of said structure and said sheet.
 6. A method of constructing anice structure at a desired offshore location in a frigid environmentwherein a base of naturally occurring ice overlies said desiredlocation, said method comprising: accumulating ice on said base byspraying water onto said base and the subsequently formed ice wherebysaid base moves downward under the added weight of the accumulated ice;continuing to accumulate ice on said base by spraying water thereonuntil sufficient ice has been accumulated on said base to form astructure which extends from the marine bottom to above the waterline;and providing protection from marginal melting of said structure whenthe ambient temperature of the water surrounding said structure risesabove the melting point of the ice forming said structure by placing asheet of plastic material having polyurethane foam on both sides thereofaround the periphery of said structure where said structure contacts thewater.
 7. A method of constructing an ice structure at a desiredoffshore location in a frigid environment wherein a base of naturallyoccurring ice overlies said desired location, said method comprising:accumulating ice on said base by spraying water onto said base and thesubsequently formed ice whereby said base moves downward under the addedweight of the accumulated ice; continuing to accumulate ice on said baseby spraying water thereon until sufficient ice has been accumulated onsaid base to form a structure which extends from the marine bottom toabove the waterline; and providing protection from marginal melting ofsaid structure when the ambient temperature of the water surroundingsaid structure rises above the melting point of the ice forming saidstructure by placing a sheet of water-impermeable material around theperiphery of said structure where said structure contacts the water,said sheet of water-impermeable material being comprised of foaminsulation trapped between two plastic sheets joined together in aquilted pattern.